The only unionized workers at Six Flags Discovery Kingdom -- its rides mechanics -- were locked out of their jobs after their shift ended Thursday afternoon and spent Friday picketing in front of the park, union and management spokespeople said.

A park spokeswoman said the union contract negotiators left management with little choice.

"We have met with the union over the course of five months during which we made significant progress on a contract," Nancy Chan said. "Over the last several weeks, the union stopped making any effort toward ratifying an agreement and we were unfortunately left with no other option to try to move the negotiations forward."

There are unionized workers at five other Six Flags parks, she said.

The picketing will continue weekends until the parties reach an agreement or the park shifts to its seven-day schedule at which time the picketing would also go seven days, union spokesman Jim Beno said.

There are 24 mechanics at the park, represented by the Oakland-based International Association of Machinists, District Lodge 190, he said.

"They were locked out by the company at 3:45 p.m. (Thursday)," Beno said. "They only voted a union in about six months ago, and we've been negotiating with the company for a contract, and it was pretty amicable until discussions of wages and some training issues came up and the company took a hard line; made a take-it-or-leave-it offer."

On-scene union spokesman Tom Brandon said park officials have created a safety issue by locking out the "highly skilled mechanics that maintain the rides within the park."

"These guys are making minimum wage for mechanics required to supply their own tools -- twice the minimum wage - and we're not sure who they have keeping the rides safe now," Brandon said. "We thought we had a deal, but then they finished their annual maintenance and rehab work (Thursday) and as soon as that was done, they locked them out."

The average wage of comparable workers in the Bay Area is closer to $36 per hour, he said. Chan said the park doesn't discuss such issues.

One mechanic was promoted to management Thursday and another worker, a welder, was certified as a mechanic, Beno said.

Park officials "have not and will never compromise the safety of our guests and employees," Chan said. "All of our rides are being inspected and maintained daily by highly trained and certified individuals with years of experience. Maintaining a safe environment is our top priority."

The striking mechanics say they're dedicated to patron safety and see their contract as a fairness issue.

"(Park management) are forcing us not to work," said American Canyon resident and three-year park employee Jeff Heineman, 30. "Sometimes you have to fight to do what you have to do, and I think getting the union in here is our fight."

Union and park officials said they're open to further negotiations and are confident an agreement will be reached.

"Our hope is that both sides will negotiate in good faith to achieve a mutually agreeable contract," Chan said. "In the meantime, we are prepared to safely operate the park as we normally would."

Though the picket line seemed to have little impact on park guests early Friday, at least one said she was concerned enough about ride safety to ask how long it had been going on.

"I'm worried about the integrity of the rides. That's why I asked how long they were locked out," Sarah Lenard of Camptonville near Grass Valley said. "If they'd said it was more than a week, I would have turned around. Let my kids on unmaintained rides? I don't think so."

Contact staff writer Rachel Raskin-Zrihen at (707) 553-6824 or rzrihen@timesheraldonline.com. Follow her on Twitter at Rachelvth.